This invention relates to improvements in screen printing machines particularly to the pallet and pallet arm constructions used in such machines.
Garments such as tee shirts are decorated using multi printing head, screen printing machines. The number of heads correspond to the number of colours to be printed. U.S. Pat. No. 3,795,189 [Jaffa] and U.S. Pat. No. 5,031,527 [Eppinger] disclose examples of the type of machines used in printing garments.
The garments are supported on a pallet, which in turn is supported, for registry with the printing head, on a pallet arm. The pallet is moved successively past the desired number of printing heads until the printing is complete. Different size garments require correspondingly different sized pallets. Usually pallets are of the same length but vary in width from a garment sleeve width to a large body width. This means that the pallets need to be removed from the pallet arms on a frequent basis.
The positioning of the pallet on its pallet arm needs to be precise and secure to ensure that registration with successive printing heads is maintained.
Conventionally one means of removably securing pallets to pallet arms has been to use 2 or 3 counter sunk screws at each end of the pallet. This is quite adequate in terms of registration accuracy and retention of alignment but involves extensive labour time to fit a new set of pallets to a machine. A number of quick release mechanisms have been proposed. Some use a key and keyhole type arrangement where either the pallet or the pallet arm has a keyhole slot and the other has a complementary waisted head that can be locked into the narrow part of the slot. Another quick release means relies on the use of a rail and complementary track arrangement with either the pallet or the pallet arm having a rail and the other having a channel track of complementary cross section to the rail and a securing means to lock the two relative to one another for registration relative to the length of the arm. In most of these devices several wing nuts or bolts need to be tightened to secure the aligned pallet to the pallet arm.
U.S. Pat. No. 4,031,825 [Jaffa] discloses a pallet which fits sleeve-like over the pallet arm and is held in place by springs in the side edges of the pallet arm. Even though the pallet appears to be simple it needs to be fabricated or machined extensively as does the pallet arm.
U.S. Pat. No. 5,592,877 in part discloses a pallet which can be installed and released quickly and utilises a pneumatic release mechanism to free the pallet. The pallet comprises two plates screwed together with the lower plate having lugs and slots that cooperate with recesses and a pin in the pallet arm to locate the pallet.
Although these quick release arrangements are an improvement they do have short comings in terms of convenience and operator simplicity. Also they all involve additional manufacturing steps in preparing the pallets.
It is desirable that a pallet be adapted for use with a vacuum system which creates a vacuum through the surface of the pallet to retain the garment in position during the printing process.
To this end the present invention provides, in a screen printing machine of the type having a pallet mounted on a pallet arm which is successively brought into registry with a series of printing heads the improvement comprising
a) a pallet having a longitudinal axis and leading and trailing edge portions extending laterally at each end of the longitudinal axis;
b) a pallet arm to support said pallet along its longitudinal axis;
c) at least one edge recess in either the leading or trailing edge of said pallet and at least one location recess in or adjacent to the other of said leading or trailing edges;
d) at least one fixed locating stop on said pallet arm adapted to fit said at least one edge recess; and
e) at least one locating means mounted on said pallet arm remote from said at least one fixed locating stop, said locating means being of complementary shape to said at least one location recess in or adjacent to said leading or trailing edge of said pallet.
The pallet of this invention is unique in that it is an integral pallet devoid of parts welded or otherwise fixed to it and having no fixing holes or recesses in its upper surface.
In contrast to prior art quick release systems, the pallet of this invention requires no additional device or protrusion to be welded mounted or adhered to its surface. The only precision machining required is to machine recesses into the edge of one end of the pallet and into the opposite edge or the base of the pallet adjacent the opposite edge. The pallets can in other respects be of the same shape and materials as is conventional for screen printing pallets.
Although two location points are adequate it is preferred to use 3 points. Preferably one adjustable point is located at the free end of the pallet arm and two adjacent the leading edge of the pallet when it is secured to the pallet arm. The recesses in the edges may be into the edge or into the top surface of the pallet at the edge. If the locating recess is into the base of the pallet it is preferably into underneath surface of the pallet adjacent the trailing edge and shaped to fit over a lug or bolt head protruding from the surface of the pallet arm adjacent its free end.
In order to press the pallet securely into its register position the surfaces of the recesses and the corresponding contact surfaces of the locating stops on the pallet arm are angled in complementary fashion so that the pallet is pressed along its longitudinal axis and also pressed onto the pallet arm. The final securing force can be provided by a spring clip or toggle clamp acting on the edge of the pallet opposite the edge which abuts the fixed stops on the pallet arm.
To fit the pallet, it is simply placed onto the pallet arm so that the locating stops are aligned with the recesses and then the spring clip or toggle is locked into place.
Only one fastening action is needed in contrast to the need to fasten several bolts in the prior art devices. Because there are no screw or bolt holes in the top surface of the pallet, a larger surface area of the pallet is available for printing than is the case for prior art pallets. Because there are no protrusions or rails mounted on the pallet as is the case with prior art quick release pallets, the pallets of this invention pack flat and are easier to store, transport and clean.
In a modification of the pallet of this invention the pallet is adapted for use with a vacuum manifold. The pallet is formed in two parts the upper surface being perforated to enable air flow through the surface and the lower surface incorporating a recess communicating with a vacuum outlet connected to a manifold. The manifold may be a third part of the vacuum pallet or it can be a part of the pallet arm which mates with the pallet.
The present invention in a further aspect provides a pallet for supporting an item to be screen printed having
a) a longitudinal axis and leading and trailing edge portions extending laterally at each end of the longitudinal axis;
b) a perforated top plate
c) a recessed bottom plate adapted for connection to a vacuum line
d) at least one edge recess in either the leading or trailing edge of one or both of said top plate and said bottom plate
e) at least one location recess in or adjacent to the other of said leading or trailing edges in either or both the top plate and said bottom plate